A jackup rig is widely used in offshore exploration for drilling wells and gas/oil production. With the increase of demand of energy, the offshore exploration is moving more and more toward the locations where hazards are present. Therefore, the operability range of a jackup rig is critical for its performance.
A jackup rig usually comprises a floatable hull with a deck or working platform, and three or four legs, where the legs' bottom is coupled with footing(s), providing support for the hull in elevated conditions. After the jackup rig arrives on location, the legs are lowered until the footing(s) touch the underneath seabed and rest on the seabed soil. A preloading is then carried out to simulate the anticipated vertical footing load during design storm and to proof test the foundation soil. During preloading, the jackup is elevated to a minimum clearance above the water and ballast water is added into the hull to impose more gravity load onto the footing. The footing will keep penetrating the seabed until the soil bearing capacity can equate the preload imposed by the footing. After a stable penetration is achieved, the ballast water is removed and the hull may then be jacked up using a jacking system to raise the working platform above the water, making the jackup rig safe to be operated in environmental conditions which impose additional loads on the jackup.
The legs of a jackup rig are commonly tubular columns or trusses, each truss leg comprising vertical chords connected with cross braces that are normally diagonally disposed. The legs normally terminate in a jackup footing that rests on the seabed. The footing provides an enlarged bearing area so as to provide an adequate bearing capacity and reduce the pressure exerted on the seabed soil. Resultantly, this reduces the penetration depth of the legs that is required by the foundation to support the jackup rig, allowing the jackup rig to be operated in a greater variety of locations and soil types with the available leg length.
Modern jackup rigs are typically equipped with individual footings, often referred to as “spudcan”, which are connected to each leg of the jackup rig. This allows the jackup rig to be supported on uneven seabeds or slopes or in the cases whereby the elevation of each leg is needed to be independently adjusted relative to the other legs. As shown in FIG. 1, a traditional spudcan 10 is typically having a generally conical upper half 11 connected to the leg A and a generally conical lower half or base 12 for contact with the seabed, where the conical upper and lower halves are usually coupled directly at their peripheral or through a spudcan side wall 13. The conical base helps ensure some penetration into the seabed, even in hard soils, so as to provide some anchoring of the legs into the seabed. Alternative to the conical shape, the upper and lower half can also consist of three or more sloping plates. The spudcan 10 further comprises a central tip or spigot 14 that is located at the bottom of the lower half 12. The central tip or spigot 14 is designed for providing a shear key for penetration in soft rock or hard soil. The conical or sloping bottom, rather than a flat base, is to help ensure the support point as concentric as possible for partial embedment case in the anticipation of not perfectly flat seabed.
During spudcan touch-down, the central tip and conical bottom will encounter gradually increasing resistance in both vertical and lateral direction as the spudcan penetrates the seabed. This is favorable as the gradual increase of resistance force allows for greater penetration into the soil, increasing the ability of the soil to absorb energy without producing large reaction forces. In addition as the spudcan penetrates further, the additional hull buoyancy is mobilized, providing an additional source of energy dissipation and further reducing the peak impact force experienced. Despite its robustness for installation, the traditional spudcan may not be able to provide the maximum fixity (the amount of rotational restraint provided by the spudcan) in dense sand or hard soil due to the conical bottom nature and the limited amount of preload that a jackup can impose. In this case, the spudcan full diameter cannot be fully utilized and the partial contact results in much lower fixity.
Skirted spudcan is normally used to increase the bearing capacity and hence fixity in dense sand compared to that of the traditional spudcans (without skirt). The skirt effectively provides an “embedment” effect which could have been achieved by forcing a flat-based spudcan (without skirt) to the same penetration level as the skirt depth. As shown in FIG. 2, a spudcan with a typical skirt 20 is similar to the traditional spudcan for having the conical upper half 21 connected to the leg A, a lower half with a single or double sloped bottom 22, a spudcan side wall 23 connecting both halves 21, 22, and a central tip or spigot 24 with additions of a peripheral skirt 25 and an internal skirt 26. For circular spudcans, the skirt height could be uniform around the entire perimeter if the spudcan bottom is of conical shape or varying in height when the lower half is formed of multiple sloping plates. Nonetheless, the lowest tip point of the normal skirt is typically lower than or at least flush with the lowest point of the central tip.
Despite its advantages in increasing the fixity for sandy seabed condition, the relatively long skirt can impose problems when the jackup encounters soft or hard seabed. For example, in soft seabed, the relatively long skirt may create leg extraction problem due to the increased frictional resistance of the skirt surface and suction effect. Furthermore, the spudcan with relatively long skirt is not suitable for hard or rock seabed. When the skirt encounters a hard layer without having the spudcan base in sufficient contact with the seabed, the skirt tip effectively becomes the support point. This would impose very high stress and potentially damage the skirt. In addition, unless the hard seabed is perfectly flat the peripheral skirt tip may only partially contact the hard layer, creating eccentric load on the spudcan. This situation is undesirable as the bending moment created by the eccentric support at the spudcan level may impose substantial initial stress at the leg even in the static condition before withstanding the design storm.
Another potential disadvantage of the spudcan with relatively long skirt could occur during lowering the jackup legs at very stiff or hard clay. Roll or pitch motions of the jackup will result in both vertical and horizontal motions of the spudcan as it impacts the seabed. Compared to the traditional conical-bottomed spudcan with a central tip, the skirt will attract a much larger horizontal resistance. Since the laterally projected width of the skirted spudcan is constant as it impacts and penetrates the seabed, the resulting horizontal resistance becomes large. The horizontal resistance may then become the controlling aspect for the utilization of the leg holding system capacity. The skirt may also damage upon impact on rock seabed.
In order to increase the stability of the spudcan for a specific soil condition, Keppel has disclosed a spudcan comprising an elongated skirt with openings at the top half of the skirt (PCT application, PCT/SG2012/000075). The tip of the elongated skirt is lower (i.e., longer) than the tip of the bottom central protrusion. Such a spudcan has advantages for installation particularly at soft clay overlying hard stratum as it allows the trapped soil to flow out through the side openings on the skirt and ensure sufficient skirt embedment into the hard stratum. However, for other soil conditions, this concept is not an ideal solution and the relatively long skirt may limit the jackup performance.